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Topic: ATX 1.2 won't drive 12V solenoid? (Read 6229 times) previous topic - next topic

ATX 1.2 won't drive 12V solenoid?

Using a relay board to power a solenoid that draws .4 amps. Relay board is powered from USB. ATX breakout works fine energizing the solenoid but board powers off when solenoid de-energizes. Tried snubber diode, big electrolitic cap, 14V MOV. I even tried shorting the poly fuse, no dice. Any ideas?

Re: ATX 1.2 won't drive 12V solenoid?

Reply #1
hi , the robot that runs this forum treatened me with expulsion if i didn't post within 24 hrs of registering.
I don't have any questions/answers at this stage , but i hope this will pass the test.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #2
Hi superUnkown, you could add a 100nF ceramic cap parallel to the relay coil.

Hope this will help

Re: ATX 1.2 won't drive 12V solenoid?

Reply #3
Similar to Vimark, but lose the diode and add a Resistor in series with the Cap and preferably use a Metallised Paper Cap with a 250V X2 rating. Worst case, you could use a Metallised Poly cap with the same rating.  Alternatively you can buy the Cap / Resistor as a single device. The Resistor should be a Carbon or Metal film, not a wirewound and at least 0.5W and use one between about 47 and 200 Ohm - I'd suggest 100 Ohm for a start.
If this helps but doesn't eliminate the problem, try a 470nF (0.47uF) cap and a 47 Ohm Resistor.

If that works, try putting the diode back in as well, but it may be too sharp a cut-off and could actually increase the problem. In that case you could again try the bigger 0.47uF cap.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #4
Oh, and put the RC close to the load.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #5
Thanks! I'll try the RC. But why would this work, but not a snubber diode? What do you think is actually causing shut down? Is it the ATX breakout board or is it the power supply? Is it the inductive spike or some sort of RF voodoo?

Re: ATX 1.2 won't drive 12V solenoid?

Reply #6
From my experience similar to this case, we've provide a small relay that in turn switch a contactor to switch on/off a whole videoke room (consist of 0.5 hp aircon, tv and a videoke machine). PC controls and its microcontroller (connected via usb) went off when the contactor+relay is turned off. Out of a few days scratching our heads on what's going on, we've decided to place an 0.47uF 500V Mylar Cap in parallel to the relay switch (not parallel to the coil). So far problem were gone.

I believe that huge current surge rushes when contactor discharges affects the controller and the PC. In your case it could be the current surge from the solenoid and not RF related.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #7
tried with various values of ceramic and electrolitic caps. Changed resistor values too. No change, ATX board shuts down when solenoid de-energizes. Guess next thing is to head down to the store on Tuesday (thanksgiving holiday here on Monday) to buy some faster response caps?

Re: ATX 1.2 won't drive 12V solenoid?

Reply #8
rooted thru junk bin and got few 200V poly caps .22uF. ran 2 in para. no dice. ran 2 in para w/ resistors (various). no dice.
So, we are all in agreement, the inductive voltage spike is what is causing the board to shut down?

Re: ATX 1.2 won't drive 12V solenoid?

Reply #9
Rookie mistake! I removed the 4007 and installed a 5817 schottky on the ATX terminals. No dice. Then I thought "I don't see what difference it'll make, but I might as well TRY  the diode close to the solenoid". (The soleniod leads are only 24" long.)
BINGO! Works like a hot damn!
Thanks guys!

Re: ATX 1.2 won't drive 12V solenoid?

Reply #10
You nailed it! :) Kill the problem from its root. Nice work.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #11
The difference is that there is 2 x 24" of antenna (and inductor) hanging between the Diode clamp and the Solenoid and lot of energy racing down those wires that is changing at a great rate of knots (a dirty great spike).The Diode is a crude, but often effective way to clamp it (in the reverse direction), but it's also a bit like stopping a car by using a solid brick wall, it's a very sudden stop with a big hit of energy and only in one direction, so ringing still occurs in the other direction until the energy dissipates. The idea of the RC is to reduce the Rate of Change (Dv/Dt) in a more controlled manner and dissipating the energy in the Res as it goes - a more elegant solution and is effective with the Diode also.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #12
Cool! Thanks for the thorough explanation, it's not voodoo after all! Now I *really* want a scope.

Re: ATX 1.2 won't drive 12V solenoid?

Reply #13
Yep, it's no more Voodoo than a Car Ignition Coil is when the points (or electronic equivalent) opens, that's how they can generate 10-20kV odd from a 12V battery. A common 12V relay coil can, depending upon the unit, generate a 300V kick or more. Just think of it like letting go of a stretched rubber band. If all that stored energy is let go at once, it packs quite a wallop.